It is desirable for an LCD to have a wide viewing angle so that an image viewed from an off-axis position appears identical to said image when viewed from an on-axis position. In order to improve the wide-view performance of LCDs, several technologies have been developed. Displays have been produced with angular compensation films such as the splayed-discotic Wide-View film for Twisted Nematic (TN) displays, multi-domained pixels for Vertically Aligned Nematic (VAN) and improved electrode geometries. These developments have enabled displays with no contrast inversion problem at wide viewing angles, i.e. although the absolute luminance of a pixel may change with viewing angle, a pixel which is switched to have an on-axis luminance higher than another pixel will remain brighter at all viewing angles, and vice versa. However, the amount of variation in pixel luminance with viewing angle is still a non-linear function of the on-axis pixel luminance in most types of LCD. This has the effect that in a colour display comprising an array of pixels, each of which is composed of a plurality of colour sub-pixels, such as red, green and blue sub-pixels in an RGB stripe display for example, if the pixel is displaying a colour consisting of different luminance values of the three colour components, these different luminance values can shift by a different amount with viewing angle, resulting in a shift in the perceived colour. In essence, the off-axis luminance response has a non-linear relationship with the on-axis luminance response, thus yielding an image that varies with viewing angle. In order to minimise angular dependent colour shift, various technologies have been employed to reduce the degree of non-linearity between the off-axis and on-axis luminance responses.
U.S. Pat. No. 4,840,460, US20050219186A1, U.S. Pat. No. 6,067,063 and U.S. Pat. No. 7,079,214 describe the use of additional electronics to further divide each LCD colour sub-pixel into two (or more) sub-regions (split sub-pixel architecture). Aside from the black level, the first sub-region has a first relatively high luminance value and the second sub-region has a second relatively low luminance value. The average luminance from the first and second sub-regions of the sub-pixel yields the desired luminance of said sub-pixel. Displaying an image in this way reduces the degree of non-linearity between the off-axis and on-axis luminance responses of a sub-pixel, thus minimising angular dependent colour shifts. A disadvantage of these technologies is the reduction in maximum luminance. Maximum luminance is limited by the fact that the second sub-region never reaching its maximum possible luminance. Maximum luminance is also limited by the fact that the additional electronics required to further divide each sub-pixel reduces the overall aperture ratio of the sub-pixel.
U.S. Pat. No. 6,801,220 and U.S. Pat. No. 5,847,688 describe how the optical effect of split sub-pixel architecture can be effectively mimicked by image processing algorithms (running in software or in the LCD control electronics) that negate the need for the additional electronics required to divide each LCD sub-pixel into at least a further two sub-regions. These algorithms may be applied to any existing colour display by adjusting the luminance of whole colour sub-pixels up and down alternately, either in the spatial or temporal domain, to create the same optical effect as split sub-pixel architecture. Luminance is effectively transferred between the colour components of neighbouring pixels rendering no overall luminance change. The disadvantage of this technology is that the algorithms lower the perceived resolution of an image relative to an image that has not been processed by the algorithms.
GB2428152 and US2010214324 describe image processing algorithms (running in software or in the LCD control electronics) that exploit the non-linear relationship between the off-axis luminance response and the on-axis luminance of an LCD in order to create a privacy function. When the privacy function is activated, the image that is viewed on-axis is different from the image that is viewed off-axis.
A first conductive electrode in electrical contact with a second conductive electrode via a third resistive electrode enables a voltage gradient to be formed along the third resistive electrode. The use of this electrode arrangement for forming a voltage gradient to switch liquid crystal (LC) molecules has been disclosed in the literature. WO2005/015300A1 describes the use of a transient voltage gradient to control the direction of the electric field during LC switching in order to avoid the production of disclination lines in the LC layer. EP1484634A1 and US2011/0170030A1 describe the use a voltage gradient to spatially alter the transmission function of a window containing liquid crystal to enable an electronically controllable curtain for said window. U.S. Pat. No. 3,675,988, U.S. Pat. No. 3,741,629A, U.S. Pat. No. 4,139,278, U.S. Pat. No. 4,106,858A, U.S. Pat. No. 4,112,361A, U.S. Pat. No. 4,392,718A all disclose a non-pixelated liquid crystal display device that conveys information via the use of an analogue voltage gradient. U.S. Pat. No. 4,815,823 describes the use of a voltage gradient within each pixel of a ferroelectric liquid crystal display device. The ferroelectric liquid crystal display device is bistable and therefore inherently has only two grey-levels. However, the use of a voltage gradient as described by U.S. Pat. No. 4,815,823 enables continuous control over the transmission of light through each pixel thus enabling more than two grey levels per pixel.